Of heterogeneous nucleation

Nucleation is assumed to begin simultaneously from centers positioned at random throughout the liquid. This is more descriptive of heterogeneous nucleation by foreign bodies introduced at a given moment than of random nucleation. We shall dispense with the requirement of simultaneity below. 

The primary nucleation on the surface of the NA is a kind of heterogeneous nucleation [25]. NA is a so called heterogeneity . In this study, all nucleation is limited to the heterogeneous nucleation. The shape of a heterogeneous nucleus is assumed parallelepiped with length of a stem l and the number of stems m and n. Here /, m, and n are counted by the number of atom... 

The most important "message" of this chapter is that there is a critical supersaturation that must be exceeded before homogeneous nucleation can occur. The background given is an essential preparation for the introduction of heterogeneous nucleation. 

In the case of heterogeneous nucleation the interfacial energy needs some redefinition because the nucleus is now formed in part in contact with the solution and in part in contact with the surface of the solid substrate ... 

Enhancement of Heterogeneous Nucleation by Specific Adsorption of Mineral Constituents... 

In line with Eq. (6.20a), one could postulate for the rate of heterogeneous nucleation of CaF2 ... 

Certain acids with hydroxylic and carboxylic groups have been shown (Schwert-mann and Cornell, 1991) to induce in Fe(HI) solutions the formation of hematite because these acids may act as templates for the nucleation of hematite. These examples illustrate that a complete understanding and quantitative description of the rate of heterogeneous nucleation will have to include surface complexation and other adsorption processes. 

The effect of heterogeneous nucleation on the crystallization of isotactic polypropylene from the melt can be easily established as follows. A small amount of powdered polypropylene is well mixed with about 0.1 wt% of sodium benzoate in a mortar or by means of an analytical mill. Some of the mixture is transferred with a spatula to a microscope slide and melted at about 250 °C on a hot block. A cover slip is pressed on to the melt with a cork to obtain as thin a film as possible.The sample is held at 200-250 °C for some minutes and then allowed to crystallize at about 130 °C on the hot stage of the microscope an unadulterated polypropylene sample is crystallized in the same way. Both samples are observed under a polarizing microscope during crystallization,the difference in spherulite size between nucleated and untreated polypropylene can be seen very clearly. An ordinary microscope can also be used by placing polarizers on the condenser and eyepiece, and adjusting these to give maximum darkness. 

Valsami-Jones et al. (1998) conducted similar studies with a synthetic, microcrystalline hydroxyapatite in either a Pb or Cd buffered solution. Using AFM, hydroxypyromorphite was observed to grow epitaxially on the hydroxyapatite surface in a clear example of heterogeneous nucleation. Cd removal differed, with the likely formation of a Ca-Cd phosphate solid solution. 

Although this line of reasoning shows one of the principal features of heterogeneous nucleation, the real situation of nucleation near a dislocation line is much more complex [S. Q. Xiao, P. Haasen (1989)]. The stress field of the dislocation changes the composition of both the matrix and the precipitate, which in turn influences both yp and Agp. In view of this fact, one has to determine whether nucleation near the dislocation occurs before or after the Cottrell atmosphere around the dislocation had sufficient time to form. 

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